Cover is a key strategic consideration of X-COM. Soldiers caught outside of cover are almost certain to be struck by enemy fire. Players should rarely if ever move units into positions where they do not enjoy at least some cover.

Cover comes in two varieties, half and full. Half cover is indicated by a half-full shield icon, full by a filled-in icon. Full cover is significantly more effective than half, but half is far better than none at all.

Cover usually only applies to certain directions. Shield icons face different directions to indicate from which angles they provide cover. Any attack from behind a straight line drawn straight across the face of a shield is considered uncovered.

Flanking attacks occur when a unit is attacked from behind. Any attack which occurs from within or behind a straight line drawn perpendicular to the unit’s facing counts as flanking. Flanking attacks offer large Aim bonuses.

Overwatch is an alternative to attacking. A unit which enters Overwatch remains on alert until the beginning of its next turn. Any hostile movement through its field of view during this time triggers a single automatic reaction Shot at a small Aim penalty.

Overwatch is a vital part of battlefield control strategy. Overwatching soldiers, properly positioned, may attack uncovered and vulnerable units revealed by cautiously advancing Scouts.

Weapons expend ammunition with surprising speed. While each soldier is equipped with unlimited magazines, reloading still takes time. Reloading may be completed after a half move (not dashing) but immediately ends the turn.

Critical hits deliver double or greater damage to the target. Each weapon has a base critical hit probability. Class Abilities and foundry upgrades can improve the chances of inflicting a critical hit.

Units may either half-move or dash. Half-moves allow a second action to take place after movement, either another half move, reloading, some special abilities, or attacking/overwatch if the unit’s weapon allows firing after movement. No other actions are possible if a unit employs full-move dashing (with the exception of the Assault class’s Run & Gun ability. Dashing Units are slightly more difficult to hit with Reaction fire than uncovered units standing still or employing a half-move.

Firing a weapon ends all movement opportunities for a turn unless special abilities preclude this rule.